1) Field of the Invention
The present invention relates to a technique to improve the safety of non-aqueous electrolyte secondary cells at the time of overcharge.
2) Description of the Related Art
Non-aqueous electrolyte secondary cells, for their high energy density and high capacity, are widely used for the driving power sources of mobile appliances.
Incidentally, non-aqueous electrolytes used in the non-aqueous electrolyte secondary cells contain flammable organic solvents; when the cells should be overcharged, smoking or firing may result. This has created a need for a technique to improve the safety of non-aqueous electrolyte secondary cells at the time of overcharge.
Patent documents 1 to 7 disclose techniques related to non-aqueous electrolyte secondary cells.    Patent Document 1: WO02/015319.    Patent Document 2: WO02/059999.    Patent Document 3: Japanese Patent Application Publication No. 2002-298909.    Patent Document 4: Japanese Patent Application Publication No. 2003-59529.    Patent Document 5: Japanese Patent Application Publication No. 2004-214189.    Patent Document 6: Japanese Patent Application Publication No. 2004-327444.    Patent Document 7: Japanese Patent Application Publication No. 10-112335.
However, these techniques cannot enhance the safety of the non-aqueous electrolyte secondary cells at the time of overcharge without compromising cell properties such as the cycle characteristic and preservation characteristic.